It is known practice to dye keratin fibres, and especially human hair, with dyeing compositions containing oxidation dye precursors, generally called oxidation bases, such as ortho- or para-phenylenediamines, ortho- or para-aminophenols and heterocyclic compounds. These oxidation bases are colourless or weakly coloured compounds which, when combined with oxidizing products, are able to produce coloured compounds by a process of oxidative condensation.
It is also known that the shades obtained with these oxidation bases can be varied by combining them with couplers or coloration modifiers, the latter being selected more particularly from aromatic meta-diaminobenzenes, meta-aminophenols, meta-diphenols and certain heterocyclic compounds such as indole compounds.
The variety of molecules used as oxidation bases and couplers allows a wide range of colours to be obtained.
The “permanent” coloration obtained by virtue of these oxidation dyes is required, moreover, to meet a certain number of demands. Thus it should have no toxicological drawbacks, it should allow shades to be obtained in the desired intensity, and it should be highly resistant to external agents such as light, bad weather, washing, permanent wave treatments, perspiration and rubbing.
The dyes must also allow grey hair to be covered, and they must be as unselective as possible, i.e. they must make it possible to produce the smallest possible colouration differences along the same lock of keratin fibre, which is generally differently sensitized (i.e. damaged) between its end and its root.